Synergistic effects of natural ventilation and animal disturbance on oxygen transfer, pollutants removal and microbial activity in constructed wetlands

This study investigated the feasibility of combining natural ventilation and animal disturbance in constructed wetlands (CWs) and the joint effects on oxygen transfer, microbial activity, organics, and nitrogen removal. The results showed that natural ventilation extended the habitat depth of earthworms by approximately 10 cm by significantly improving oxygen transfer in CWs; in turn, the earthworms slightly promoted the addition of oxygen inside CWs through burrowing activity. Therefore, the interaction between natural ventilation and animal disturbance in CWs mutually reinforced oxygen transfer, enzymatic activity, and the ammonification, nitrification, and aerobic degradation of organics. Additionally, the combination of natural ventilation and animal disturbance in CWs promoted the oxygen transfer rate by 42.1%-68.2%; promoted catalase, urease, and dehydrogenase activity by 19.3%-24.8%, 17.4%-22.3%, and 18.1%-25.6%, respectively; and promoted COD and NH3-N removal loads by 48.6%-74.2% and 94.9%-135.3%, respectively. To achieve higher total nitrogen removal, moderate wind speeds (<= 1 m/s in this study) are recommended to simultaneously create aerobic and anoxic/anaerobic conditions. Although natural ventilation reduced the microbial diversity in CWs by promoting the abundance of aerobes, the combination of natural ventilation and animal disturbance was generally conducive to improving microbial diversity. The relationship between wind speed and oxygen transfer rate and COD and NH3-N removal loads in naturally ventilated CWs conformed to cubic equations.